Janet Browne opens the second volume of her biography of Darwin in 1858, at the point when he received Alfred Russel Wallace's letter from the East Indies, containing Wallace's theory of natural selection and the development of new species. This alarmed the slowly cumulative writer of what was to become The Origin of Species, who was ashamed of his "trumpery feelings" of disappointment that his "priority" of discovery was compromised. Browne describes beautifully the gentlemanly way in which Darwin and his friends orchestrated the revelation of the theory, in a discussion that credited both scientists. As she rightly says, the friendship and respect that developed between Wallace and Darwin is a credit to both men - and it survived Wallace's later eccentric forays into spiritualism.

Wallace was a solitary explorer and wanderer. Darwin was a recluse who lived in the midst of web within web of intersecting social relationships. Browne emphasises the role of the postal system - "the pre-eminent collective enterprise of the Victorian period" - in Darwin's research, in his friendships and support systems as the social and cultural effects of The Origin proliferated, and in his family life. Darwin corresponded with "civil servants, army officers, diplomats, fur-trappers, horse-breeders, society ladies, Welsh farmers, zookeepers, pigeon-fanciers, gardeners, asylum owners and kennel hands", sending letters all over the globe and spending the equivalent of £2,000 by 1877 on postage and stationery. Browne gives an excellent idea of the hum of all this exchange of ideas and information.

She gives also a thorough and lively image of many of the people involved, from Darwin's mentors Lyell and Hooker, to the wanderers, Wallace and the delightful Henry Walter Bates, who explored the Amazon and whose work on butterflies provided an example of the observed formation of new species. She shows how Darwin came to appreciate TH Huxley's wit as his book was attacked by friends and competitors such as Richard Owen. She makes less of the religious doubts of Emma Darwin or even Darwin himself than other biographers, and is matter-of-fact about Huxley's anti-religious brilliance. His epigrams still glitter. "Extinguished theologians lie about the cradles of every science as the strangled snakes besides that of Hercules." The tone of the new Natural History Review, he told Darwin, would be "mildly episcopophagous". And what he actually said on the spur of the moment to Bishop Wilberforce, comparing a "miserable ape" favourably to a powerful and brilliant man who used his gifts "for the mere purpose of introducing ridicule into a scientific discussion", is funnier and sharper than the monkey-bishop joke to which it has been reduced.

Browne is interested in the spread of information - she tells us of Mudie's circulating library's interest in spreading the ideas of The Origin and of its competitors. She has a particularly interesting (and to me new and shocking) section on the liberties taken by Darwin's early translators, who seem to have felt themselves quite free to write polemic of their own into the text, introducing religion or Lamarck, leaving out sentences they disagreed with and adding chapters of their own.

What sort of a man was Darwin, as seen by this biographer? She understands both his country gentleman's reclusiveness and his intellectual sociability. She characterises his style as one of "artless intimacy", comparing it to that of Gilbert White and Sterne, saying that Darwin "spontaneously tapped into well-known and unthreatening literary genres". The style, she says, is the man, "a reputable scientific gentleman, courteous, trustworthy and friendly ... a champion of common sense ... and scornful of mere conjecture". He wrote in 1859: "If I know myself, I work from a sort of instinct to try to make out truth." His son Francis, much later, described his mind at work. "It was as if he were charged with theorising power ready to flow into any channel on the slightest disturbance, so that no fact, however small, could avoid releasing a stream of theory." Browne gives details of his compulsive, endlessly thorough work on facts and ideas. She is particularly good on his efforts to understand the evolutionary adaptations of orchids, or the behaviour of roots under the earth.

Like many other modern commentators, she is baffled by Darwin's recurrent illnesses, which included bouts of vomiting and diarrhoea, as well as skin eruptions. She is not inclined to accept the idea that they were the result of tropical infections, caught on the Beagle, and is tempted to see them as a convenient device for allowing him to refuse invitations to speak, to stay at home and to go on working steadily. Something feels wrong with this - Darwin was too much of a scientist and too much of a workaholic to have succumbed to a purely psychosomatic disorder. And he does seem to have been really very ill, very frequently. It is clear that the illness did allow him to make a pattern of daily life that included work, when possible, and excluded social disruptions.

Like other modern commentators also, Browne is concerned to avoid the Victorian belief that a biography is a study of a Great Man, and to emphasise the ways in which Darwin's work was limited by its time and place. She emphasises the way in which his work was a collaboration with all his informants, not only the major scientists but the minor pigeon-fanciers and worm-diggers. We do not like to read about people who could do things no one else could have done - yet Darwin's endless application and charge of theorising power have been available to few others.

More seriously, and I think without quite meaning to, she falls into the trap of judging Darwin because he was a Victorian gentleman, a member of a family which was both "well bred" and had produced more than its statistical share of what his cousin, Francis Galton, called "hereditary genius". She describes his discussion of sexual selection in The Descent of Man, which treats female sexual selection in animals but not in humans, comments reasonably that "for Darwin it was self-evident that in civilised regimes men did the choosing" and concludes slightly unfairly that "for him, Victorian males set the evolutionary compass". She omits any discussion of The Expression of the Emotions in Man and Animals in which, as its editor Paul Ekman shows, Darwin is interested in cross-cultural, and cross-species universal characteristics, not in hierarchies.

When Marx first read The Origin of Species he commented: "Darwin discovers among beasts and plants the society of England, with its division of labour, competition, opening up of new markets, inventions and the Malthusian struggle for existence." He went on to criticise the "clumsy English style" and to say that the book provided a basis in natural history for his and Engels's view. There has been a tendency among Marxist, or marxisant, critics of Darwin, and social Darwinism, to criticise, or ridicule, the theory as a simple product of the society in which it was developed. This tendency also exists among post-Christian instinctive egalitarians. Browne reinforces this by including Darwin's own successful prudence as an investor as a characteristic that confines him to 19th-century capitalism in other areas too - and limits his understanding.

These views tell us as much about our own priorities and problems as about Darwin and his work. Marx is tendentiously metaphorical - where are opening markets in Darwin's book? JS Mill's enthusiasm is intellectual, not social. "Mr Darwin's remarkable speculation on the origin of species is another unimpeachable example of a legitimate hypothesis ... he has opened up a path of inquiry full of promise, the results of which none can foresee." Like Freud and Marx, Darwin has suffered from becoming a belief system, when he was simply a very original thinker. A legitimate hypothesis is not an article of faith. Darwin's path of inquiry is still open, still providing knowledge and ideas.